JP3377428B2 - ERW steel pipe for steel towers having excellent hot-dip galvanizing crack resistance and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric resistance welded steel pipe for a power transmission tower, especially a large diameter and thick walled steel tower used for a large-sized steel tower, and a method for producing the same. More specifically, it is excellent in hot dip galvanizing crack resistance. The present invention relates to an electric resistance welded steel pipe for a steel tower and a method for manufacturing the same.

[0002]

2. Description of the Related Art Among steel pipes used for power transmission towers, UOE steel pipes are used as large-diameter thick-walled steel pipes for large-scale steel towers. Further, hot-dip galvanizing is used as the rust preventive means regardless of the size of the steel pipe. As is well known, hot-dip galvanizing is a method in which an object is immersed in hot-dip zinc at about 450 ° C. to perform plating, and in the case of steel pipes for steel towers,
This hot dip galvanizing is performed on the welded steel structure after welding various attachment members such as flanges to the steel pipe.

In hot dip galvanizing of a welded steel structure, intergranular cracking may occur mainly in the weld heat affected zone (hereinafter referred to as the HAZ portion), and in the case of steel pipes for steel towers, the HAZ portion accompanying welding of attachment members The cracking in is a problem. This hot-dip galvanizing crack is caused by the intrusion of liquid zinc into the grain boundaries of the HAZ portion that has been coarsened, and the residual stress of the welded portion and the thermal stress generated during bath immersion, which generally occur more often in high-strength materials. The HAZ part has a hardness of 260
If it exceeds -270 Hv, the cracking sensitivity becomes extremely high.

[0004] Regarding hot dip galvanizing cracks in welded steel structures, stress and materials that are the factors have been widely studied. As a method for preventing the cracks, there is a reduction in residual stress at the welded portion, a bath dipping method with less thermal stress, Research and development has been conducted on the shape of structures. Further, from the viewpoint of materials, steels and the like have been proposed in which the amount of alloying elements is regulated to improve the hot-dip galvanizing crack resistance (Japanese Patent Publication No. 2-5814).

[0005]

By the way, for steel pipes for large-diameter and thick-walled steel towers, UOE steel pipes have been used so far, but recently, electric resistance welded steel pipes which are cheaper in pipe manufacturing cost than UOE steel pipes. The conversion to is partly underway. The applicant started this conversion relatively early and has been conducting various studies since then, but in the process, it was found that large-diameter thick-walled ERW steel pipes for steel towers have the following problems.

Large-diameter thick-walled ERW steel pipes for steel towers are also made of hot-rolled steel strip as a material for pipe forming-ERW welding-Welding bead removal processing-Welding portion heat treatment-Reduction by sizer. It is manufactured by each step of pipe processing, but the size is larger in diameter and thickness than a normal ERW steel pipe, and for the manufactured ERW steel pipe, after welding of various mounting members is performed, Hot dip galvanizing is performed for rust prevention.

However, large-diameter thick-walled ERW steel pipes for steel towers are
Despite the fact that the vicinity of the electric resistance welded part including the electric resistance welded HAZ part is softened by heat treatment, there is a risk of plating cracks in the vicinity of the electric resistance welded part of the steel pipe body when hot dip galvanizing is performed. There was found. UOE with the same welded steel pipe
In the case of a steel pipe, cracks may occur in the HAZ portion due to welding of the mounting member, but plating cracks do not occur in the steel pipe body side including the vicinity of the seam welded portion. Even with the same electric resistance welded steel pipe for steel towers, this plating crack does not pose a problem in the case of a small medium diameter pipe that is the original electric resistance welded pipe size. From these facts, it is considered that the hot dip galvanizing crack near the electric resistance welded portion is a phenomenon peculiar to the large-diameter thick ERW steel pipe for steel towers.

[0008] Although the conventionally considered countermeasures against plating cracks are effective against cracks in the HAZ portion due to welding of mounting members, electric resistance welding peculiar to large-diameter thick-walled electric resistance welded steel pipes for steel towers It has no effect on hot dip galvanizing cracks that occur in the vicinity of welds.

An object of the present invention is a steel tower having excellent hot-dip galvanizing crack resistance, which exhibits excellent resistance to hot-dip galvanizing cracks in the vicinity of the electric resistance welded portion peculiar to large-diameter thick ERW steel pipes for steel towers. An object of the present invention is to provide an electric resistance welded steel pipe and a manufacturing method thereof.

[0010]

In order to achieve the above object, the inventors of the present invention have a phenomenon, a cause, of a hot dip galvanization crack near an electric resistance welded portion peculiar to a large-diameter thick ERW steel pipe for a steel tower, Furthermore, we conducted a research study on countermeasures. As a result, the following findings were obtained.

The electric resistance welded steel pipe is manufactured by using the hot rolled steel strip as a raw material in the following steps: pipe forming-electric resistance welding-welding bead removal processing-weld portion heat treatment-contractor processing using a sizer. In the production of such an electric resistance welded steel pipe, since the upset is performed by electric resistance welding, as shown in FIG. 1A, the vicinity of the electric resistance welded portion swells mainly on the outer surface side of the steel pipe 1 to form the weld bead 2. Occurs,
This welding bead 2 is cut and removed by a bite or the like in a step following the electric resistance welding. However, the steel pipe that advances along the electric resistance welded pipe line inevitably causes meandering and the like. As a result, in the actual pipe manufacturing, as shown in FIG.
The stepped portion 3 caused by the uncut portion is intermittently generated in the longitudinal direction of the steel pipe.

The steel pipe on which the step portion 3 remains is finally subjected to a shrink pipe forming process by a sizer. At this time, if the height h of the step portion 3 is large (although it is large, it is usually 0.5 mm or less, 0 .2 to 0.3 mm), FIG. 1 (b)
As shown in (c), the convex portion formed by the step portion 3 is strongly pressed cold during processing, and stress concentrates on the hatched portion 4 ′ from the lower portion of the step portion 3 to the lower portion thereof. ′ Causes work hardening, and the hot-dip galvanizing crack susceptibility of this work hardened portion 4 increases. In fact
The hot dip galvanizing cracks in the vicinity of the electric resistance welded portion, which is peculiar to the large-diameter thick ERW steel pipe for steel towers, is a phenomenon that occurs correspondingly to the positions corresponding to both sides of the weld bead 2 in a positional manner. It has been confirmed.

[0013] Among the electric resistance welded steel pipes for steel towers, this cracking characteristically occurs in the large-diameter thick-walled pipes because a large-diameter thick-walled pipe (external Diameter 45
7 mm or more, wall thickness 14 mm or more) and high strength (strength of 50 kgf / mm ² or more), the rolling force received during sizer molding is large, the upset amount during electric resistance welding is large, and the welding beads 2 and The height of the stepped portion 3 is essentially large, the meandering of the electric resistance welded steel pipe becomes noticeable as the outer diameter of the pipe increases, and the height of the stepped portion 3 tends to increase. It is thought that the cause is remarkable hardening. On the other hand, the fact that hot-dip galvanizing cracks do not occur in UOE steel pipes, which are conventional large-diameter thick-wall steel pipes for steel towers, is because this type of steel pipe is not subjected to contraction pipe forming by a sizer.

The base metal strength of the electric resistance welded steel pipe is affected by the C content in the steel, and when the C content in the steel is 0.12% or more, the susceptibility to hot dip galvanizing cracks near the electric resistance welded portion increases.

The hardening phenomenon due to cold working in the sizer molding is a problem for welded steel structures.
Electric resistance HA before hardening phenomenon of Z part and heat treatment of welded part
Although it has a completely different mechanism from the hardening phenomenon of the Z part, as far as the quantitative effect of the hardness on the hot dip galvanizing cracks is concerned, the same result as the hardening phenomenon of the HAZ part is obtained, and the hardening When it exceeds 260-270 Hv, the hot-dip galvanizing crack susceptibility is significantly increased.

As described above, in a large-diameter thick-walled electric resistance welded steel pipe for a steel tower, the reduction of the stepped portion at the sizer causes work hardening, which in turn increases the susceptibility to hot dip galvanizing cracks.
The one in which the step portion is removed before the sizer molding has excellent hot-dip galvanizing crack resistance. In addition, even if the sizer is not removed and subjected to sizer molding, but the work-hardened part generated by sizer is removed, the hot dip galvanizing crack resistance is excellent. Furthermore, as a countermeasure in the pipe manufacturing process, it is effective to remove the stepped portion before the sizer molding and to remove the work-hardened portion generated by the sizer molding after the sizer molding.

The present invention has been completed based on the above findings, and the gist of the following is an electric resistance welded steel pipe for a steel tower and a method for manufacturing the same.

[0018]

The first electric resistance welded steel pipe for a steel tower; the amount of C in the steel is 0.12% or more by weight, and the outer diameter is 45.
A large-diameter, thick-walled electric resistance welded steel pipe for a steel tower having a wall thickness of 7 mm or more and 14 mm or more, which is subjected to a first surface treatment for removing the electric resistance weld bead portion, and after the first surface treatment. An electric resistance welded steel pipe for a steel tower having excellent resistance to hot dip galvanization cracking, which has undergone a second surface treatment that removes a step portion that partially remains near the processed portion.

Second electric resistance welded steel pipe for steel towers; the C content in steel is 0.12% or more by weight, and the outer diameter is 45.
A large-diameter, thick-walled electric resistance welded steel pipe for a steel tower having a wall thickness of 7 mm or more and 14 mm or more, which is subjected to a first surface treatment for removing the electric resistance weld bead portion, and after the first surface treatment. For a steel tower excellent in hot dip galvanizing cracking resistance, which is subjected to a second surface treatment that removes a work-hardened portion generated in the vicinity due to a step portion partially remaining in the vicinity of the processed portion. ERW steel pipe.

First manufacturing method: Pipe forming-electric resistance welding-welding bead removal processing-weld portion heat treatment using a hot-rolled steel strip having a C content in the steel of 0.12% or more by weight as a raw material. -Outer diameter is 457 mm due to each process of shrink pipe processing with sizer
As described above, in the method for producing a large-diameter thick-walled electric-resistance-welded steel pipe for a steel tower having a thickness of 14 mm or more, the electric-resistance welding is performed at a stage after the electric-resistance welding weld bead removal processing is finished and before the shrink pipe processing with the sizer A method for producing an electric resistance welded steel pipe for a steel tower, which comprises cutting or grinding a surface step remaining in the vicinity of a weld bead removal processed portion to 0.1 mm or less.

Second production method: Pipe forming-electric resistance welding-weld bead removal processing-weld portion heat treatment using a hot-rolled steel strip in which the C content in steel is 0.12% or more by weight. -Outer diameter is 457 mm due to each process of shrink pipe processing with sizer
As described above, in the method for producing a large diameter thick-walled electric resistance welded steel pipe for a steel tower having a thickness of 14 mm or more, at least the surface in the vicinity of the electric resistance welded bead removal processed portion is treated at the stage after the shrink pipe processing with the sizer is performed. A method for producing an electric resistance welded steel pipe for a steel tower, which comprises cutting or grinding to a depth of 0.1 mm or more.

[0023]

In the present invention, the C content of the raw steel is 0.1
The reason for limiting the content to 2% or more is that if it is less than 0.12%, the work hardening of the cold-worked portion is relatively small, and hot-dip galvanizing cracks near the electric resistance welded portion due to sizer molding are unlikely to occur. In addition, the outer diameter of the electric resistance welded steel pipe is 457 mm or more, and the wall thickness is 1
The reason why the size is limited to 4 mm or more is that the upset amount at the time of electric resistance welding becomes large within this size range, the welding beat and the step portion become high, and the rolling force at the time of forming the sizer becomes essentially large. This is because hot dip galvanizing cracks near the electric resistance welded portion due to forming are likely to occur. A particularly effective dimensional range is an outer diameter of 500 mm or more and a wall thickness of 14 mm or more.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below.

In the electric resistance welded pipe line, a thick and wide hot-rolled steel strip made of carbon steel having a carbon content of 0.12% or more, which is used as a steel plate for steel pipes for large towers, is cut into a predetermined width.
A large diameter thick-walled steel pipe is formed by electro-welding the butt edges while continuously forming a circular pipe. At this time, welding beads are generated in the vicinity of the electric resistance welded portion [see FIG. 1 (a)].

The steel pipe after the electric resistance welding is continuously sent to a bead removing device, where the welding bead is cut and removed by a bite or the like. At this time, due to the meandering of the steel pipe etc., the uncut portion of the weld bead occurs, and the stepped portion due to this causes the steel pipe to be near the bead removal processing part, specifically, the part corresponding to both sides of the weld bead. Occurs intermittently in the longitudinal direction [Fig. 1
(B)].

The steel pipe subjected to bead removal processing is subjected to heat treatment at the welded portion and contracted pipe forming processing with a sizer to be a large diameter thick-walled electric resistance welded steel pipe for a steel tower. However, the stepped portion is pressed down by the sizer molding, and the work-hardened portion resulting from this is a cause of increasing the susceptibility to hot dip galvanizing.

Therefore, in the present invention, the step portion is removed before the sizer molding. Methods for removing the stepped portion include cutting with a cutting tool, grinding with a velder (grinder), and cutting or grinding with other mechanical means, but crushing is not included. Crushing is accompanied by work hardening and becomes a cause of increasing susceptibility to hot dip galvanizing cracking.

Even if there is a step, the height is 0.1.
When it is less than or equal to mm, work hardening due to reduction in sizer molding is slight, and its microhardness does not exceed 260 Hv, so it does not become a cause for increasing hot-dip galvanizing crack susceptibility. Therefore, the stepped portion of 0.1 mm or less may be left unattended, while the height of the stepped portion needs to be 0.1 mm or less when removing the stepped portion. Further, since the work hardening of the step portion having the inclination angle θ of less than 45 ° is relatively slight, it may be left.

By performing such step removal processing before sizer molding, work hardening due to reduction of the step is avoided in sizer molding, and hot dip galvanizing cracks in large-diameter thick ERW steel pipes for steel towers are avoided. The cause is removed. A method for producing an electric resistance welded steel pipe including this step removing process is the first production method of the present invention, and an electric resistance welded steel pipe having improved hot dip galvanization cracking resistance of the present invention is subjected to this processing. This is the first ERW pipe.

The step of removing the step may be performed before or after the heat treatment of the welded portion as long as it is before the sizer molding. By the way, the weld heat treatment is at least HA of the steel pipe.
After heating the vicinity of the weld including the Z part to 3 or more points of Ar,
It is carried out by cooling to c1 point or less by air cooling or more, then reheating to a temperature of 550 ° C. or more and Ac1 point or less as necessary, and then cooling at a cooling rate of air cooling or more.

Further, in the present invention, instead of the step removing process before the sizer molding, or in addition to the step removing process as required, the cold work hardening part caused by the reduction of the step after the sizer molding is removed. It is also possible to do so. This hardened part is removed mechanically .

Mechanical methods include surface cutting with a cutting tool and surface grinding with a velder (grinder). This surface cutting or grinding does not have to be performed on all surfaces,
It may be performed at least for the work hardening part. Further, it has been confirmed that the range in which the step portion is sizer-formed to cause work hardening (260 Hv or more), which is a problem in plating cracking, is about 0.1 mm in the depth d in the thickness direction from the surface. Therefore, the surface cutting margin needs to be 0.1 mm or more. However, excessive shaving reduces the wall thickness of the steel pipe and saturates the effect. Further, as will be described later, the hardened portion in the deep layer does not cause plating cracks. The desired cutting allowance is 0.
It is 15 to 0.3 mm.

[0035]

By carrying out such hardened portion removing processing after the sizer molding, the work hardened portion which causes hot dip galvanizing cracks in the large-diameter thick ERW steel pipe for steel towers is removed, and the cracking susceptibility is lowered. . ERW manufacturing method comprising a process of processing a second manufacturing method of the present invention. Further, an electric resistance welded steel pipe having improved resistance to hot dip galvanizing cracking by undergoing this processing is the second electric resistance welded steel pipe of the present invention.

Then, such steps and hardened portions are removed over the entire length of the steel pipe, and the micro hardness of the portions corresponding to both sides of the electric resistance weld bead is 0.2 mm at least from the outer surface in the thickness direction. In the above range, by setting the steel pipe to 260 Hv or less over substantially the entire length of the steel pipe, the hot dip galvanizing crack near the electric resistance welded portion becomes complete.

If there is a portion having a micro hardness of more than 260 Hv, this portion becomes the starting point of hot dip galvanizing cracks. Further, the elongation in the bath (defined in FIG. 2), which is an index of hot-dip galvanizing cracking resistance, is 2% or less in this portion. A desirable micro hardness is 250 Hv or less. The hardness is 100-500 g using a Vickers hardness tester.
It is a value measured in degree.

Although the average hardness of the entire steel pipe depends on the strength level, it is 200 to 220 Hv in the 55 kg class and up to about 230 Hv in the 60 kg class, and the hardness of the electric resistance welded HAZ part is also affected by the softening heat treatment. It is controlled to a certain degree of hardness. In the conventional large-diameter thick ERW steel pipe for steel towers, despite softening heat treatment near the weld,
A local work-hardened portion of more than 260 Hv due to a step portion due to the uncut portion of the bead portion was formed in a portion corresponding to both sides of the weld bead, and this might cause hot dip galvanization cracking. .

The occurrence of hot dip galvanizing cracks in a large-diameter thick ERW steel pipe for steel towers is limited to the outer surface of the steel pipe. This is because, on the outer surface side, the residual stress of the electric resistance welded steel pipe and the thermal stress at the time of hot dip galvanizing become tensile stress, which accelerates the generation and propagation of cracks. Also, this crack is
It is also caused by the fact that molten zinc penetrates into the grain boundaries from the surface of the steel pipe.

From these facts, except for the outer surface which is in contact with molten zinc, the influence is up to 0.2 mm at the depth in the thickness direction, and the depth in the thickness direction from the outer surface is affected. If the micro hardness is 260 Hv or less in the range of 0.2 mm, it is sufficient from the viewpoint of hot-dip galvanizing crack resistance. In other words, even if the hardness exceeds 260 Hv on the inner surface side exceeding 0.2 mm from the outer surface, the hot-dip galvanizing crack resistance is not impaired. Therefore, the micro hardness is 2
The range defined as 60 Hv or less is at least 0.2 mm in the thickness direction from the outer surface.

[0042]

EXAMPLES Next, examples of the present invention will be shown, and the effects of the present invention will be clarified by comparison with comparative examples.

Using a hot-rolled steel strip having the composition shown in Table 1, as a raw material, for steel towers having the specifications shown in Table 2 by pipe forming-electric resistance welding-welding bead removal-weld portion heat treatment-condensation pipe with a sizer A large-diameter thick ERW steel pipe was manufactured. In all of the electric resistance welded steel pipes, the C content in steel is 0.12% or more and the outer diameter is 45
It is 7 mm or more and the wall thickness is 14 mm or more.

At this time, the bead position for welding the bead is changed stepwise by the unit length to intentionally generate the uncut portion, thereby forming stepped portions of various heights, and by the unit length. The maximum height of the stepped portion was defined as the stepped height. A part of the steel pipe in which the step portion was formed was subjected to surface grinding with a velder before the step of forming the sizer. The steel pipe after the sizer molding was cut into unit lengths and divided into groups.

In one group, a tile-shaped test piece was cut out from the vicinity of the welded portion as it was, expanded so as not to apply a reduction to the outer surface near the welded portion, and then the outer surface was left as it was.
The tensile test piece shown in FIG. 3 was sampled. Then, the maximum microhardness of the outer surface of the tensile test piece was measured, and the test piece was subjected to a tensile test in a distilled zinc bath to examine the elongation in the bath defined in FIG. The hot-dip galvanizing crack resistance was considered good.

In another group, after molding with a sizer, the outer surfaces on both sides of the welded portion are each about 20 by a velder.
By performing full length grinding in the pipe length direction with a width of mm, the work hardened portion formed by pressing down the step portion by sizer molding was mechanically removed, and the same investigation was conducted.

[0047]

Incidentally, the weld bead width w (see FIG. 1)
Was 6 mm on average for Steel A and 8 mm on average for Steel B.

[0049]

[Table 1]

[0050]

[Table 2]

[0051]

[Table 3]

The results of the investigation are shown in Table 3. In Table 2 showing the steel pipe specifications, YS, TS, and Hv (5 kg) are values tested on the base metal portion after forming the steel pipe. Also, H
v (5 kg) is a value measured with a load of 5 kg, and hardening of the outer surface by cold working is not measured with this load of 5 kg.

Trial Nos. 1, 2, 21, and 22 are those in which the step difference in front of the sizer exceeds 0.1 mm and the step difference is left unattended. As a result, the surface hardness after sizer is 260H.
v, the hot-dip galvanizing resistance is poor (elongation in the bath is 2%
Less than).

Test Nos. 3 to 5 and 23 to 25 are those in which the step before the sizer exceeds 0.1 mm, but the step is ground to 0.1 mm or less. Surface hardness after sizer is 260
It was Hv or less, and the hot dip galvanizing resistance was good (elongation in the bath was 2% or more).

The trial numbers 6 and 26 are trial numbers 1, 2, 21 and 22.
Similar to, the step in front of the sizer exceeds 0.1 mm, and
This step is left unattended. The surface hardness after the sizer exceeded 260 Hv, and the hot dip galvanizing resistance was poor.

The trial numbers 7 and 27 have a step difference of 0.1 in front of the sizer.
Since the diameter exceeded 0.1 mm, the surface was velder ground after the sizer, but the grinding amount did not reach 0.1 mm. The surface hardened layer due to the step was not sufficiently removed, and as a result, a portion having a surface hardness of more than 260 Hv remained, and the hot dip galvanizing resistance became poor.

In the test Nos. 8 to 10 and 28 to 30, the step before the sizer exceeds 0.1 mm, but the surface after the sizer is 0.
It is ground by 1 mm or more. The surface hardened layer due to the step was removed, the surface hardness after sizer did not exceed 260 Hv, and the hot dip galvanizing resistance was good.

[0058]

[0059]

As is apparent from the above description, the electric resistance welded steel pipe for a steel tower and the method for producing the same according to the present invention are characterized by the melting characteristically occurring in the vicinity of the welded portion of the large diameter thick wall electric resistance welded steel pipe used in the steel tower. By preventing galvanizing cracks, it will be possible to apply electric resistance welded steel pipes to large steel towers, which will reduce the construction cost of large steel towers.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing a mechanism of generation of a step portion which causes hot-dip galvanizing cracks and a hardened portion caused by the step portion.

FIG. 2 is an explanatory view of elongation in a bath, which is an index of hot-dip galvanizing cracking resistance.

FIG. 3 is a dimensional drawing of a test piece for investigating elongation in a bath.

[Explanation of symbols]

1 steel pipe 2 weld beads 3 steps 4 curing section

Front page continued (72) Inventor Kinya Kawabata 3-3-22 Nakanoshima, Kita-ku, Osaka City, Osaka Prefecture Kansai Electric Power Company (72) Inventor Takahiro Kushida 4-53-3 Kitahama, Chuo-ku, Osaka City, Osaka Sumitomo Metal Industry Co., Ltd. (72) Inventor Yasushi Kajiwara, 1850 Minato, Wakayama, Wakayama Sumitomo Metal Industries, Ltd.Wakayama Steel Works (72) Nobuyuki Kusou, 1850, Minato, Wakayama, Wakayama Sumitomo Metal Industries, Ltd. 56) References JP-A-6-136481 (JP, A) JP-A-8-144008 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] 1. The amount of C in steel is 0.12% or more by weight,
A large-diameter, thick-walled electric resistance welded steel pipe for a steel tower having an outer diameter of 457 mm or more and a wall thickness of 14 mm or more, which has undergone a first surface treatment for removing an electric resistance weld bead and also has a first surface treatment. An electric resistance welded steel pipe for a steel tower having excellent resistance to hot-dip galvanizing cracking, which is characterized by being subjected to a second surface treatment in which a step portion partially remaining near the processed portion is removed. 2. The C content in steel is 0.12% or more by weight,
A large-diameter, thick-walled electric resistance welded steel pipe for a steel tower having an outer diameter of 457 mm or more and a wall thickness of 14 mm or more, which has undergone a first surface treatment for removing an electric resistance weld bead and also has a first surface treatment. Hot dip galvanizing, characterized by being subjected to a second surface treatment for removing a work-hardened portion generated in the vicinity of the processed portion due to a step portion partially remaining in the vicinity of the processed portion. ERW steel pipe for steel towers with excellent crackability. 3. Pipe forming-electric resistance welding-welding bead removal processing-weld zone heat treatment-by sizer, using a hot-rolled steel strip having a C content in the steel of 0.12% or more by weight as a weight ratio. In a method for producing a large-diameter thick-walled electric-resistance welded steel pipe for a steel tower having an outer diameter of 457 mm or more and a wall thickness of 14 mm or more by each process of reducing pipe processing, after the electric-resistance welding weld bead removing process is completed, reduction by a sizer is performed. A method for producing an electric resistance welded steel pipe for a steel tower, which comprises cutting or grinding a surface step remaining in the vicinity of a portion where an electric resistance welded bead is removed to a depth of 0.1 mm or less before the pipe is formed. 4. A pipe forming-electric resistance welding-welding bead removing process-weld portion heat treatment-by using a sizer, using a hot-rolled steel strip having a C content in steel of 0.12% or more by weight as a weight ratio. In the method for manufacturing a large diameter thick-walled electric resistance welded steel pipe for a steel tower having an outer diameter of 457 mm or more and a wall thickness of 14 mm or more by each step of the shrinking process, after performing the shrinking process with a sizer, A method for manufacturing an electric resistance welded steel pipe for a steel tower, which comprises cutting or grinding at least a surface of a portion near an ERW weld bead removing processed portion to a depth of 0.1 mm or more.